This invention relates generally to turbofan gas turbine engines and more particularly to fan casings for such engines.
A turbofan gas turbine engine used for powering an aircraft in flight typically includes, in serial flow communication, a fan, a low pressure compressor or booster, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine. The combustor generates combustion gases that are channeled in succession to the high pressure turbine where they are expanded to drive the high pressure turbine, and then to the low pressure turbine where they are further expanded to drive the low pressure turbine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan and the booster via a second rotor shaft.
The fan includes a plurality of circumferentially spaced apart fan blades extending radially outwardly from a rotor disk that is drivingly connected to the low pressure shaft. Each fan blade generally has an airfoil section and an integral dovetail root section that attaches the blade to the rotor disk. The fan is rotatively supported on a nonrotatable frame, commonly referred to as the fan frame, by a support system that typically includes a number of bearings and bearing support structure.
During engine operation, there is a remote possibility that a foreign body, such as a bird, could impact the fan and cause a fan blade-out event; i.e., part or all of a fan blade becomes detached from the rotor disk. Such a detached fan blade could cause considerable damage to the aircraft powered by the engine if it were not contained by the fan casing. Various containment systems have been developed to prevent such damage. Fan blade containment systems are of two primary types: xe2x80x9chardwallxe2x80x9d systems which include an annular containment case manufactured from a high strength material with an adequate shell thickness to absorb the kinetic energy of an impacting fan blade, and xe2x80x9csoftwallxe2x80x9d systems which employ nesting areas defined by inner and outer annular shells having honeycomb structures disposed therein. In addition, ballistic material, such as an aromatic polyamide fiber, may be wrapped around the case structure. Blade fragments are captured in the nesting area and are thus contained within the system and prevented from further contact with other fan blades. Hardwall and softwall containment systems may also be combined.
Prior art fan casings often have provisions for the installation of acoustic panels which absorb noise, such as resonator-type absorbers which include a cellular (e.g. honeycomb) structure covered by a perforated facesheet exposed to the fan flowpath. These acoustic panels are typically installed axially forward and aft of the fan blades, while the portion of the casing axially aligned with the fan blades is designed to define the fan flowpath boundary and to serve as a containment system. In some engine designs, particularly those which do not have a low pressure compressor (or xe2x80x9cboosterxe2x80x9d), the structural configuration of the fan section provides prohibitively small axial space for the installation of these conventional acoustic panels for noise suppression.
Accordingly, there is a need for a fan casing design which combines the functions of blade containment and noise suppression into a limited physical space.
The above-mentioned need is met by the present invention, which provides a fan casing for a gas turbine engine that combines the functions of blade containment and noise suppression. The fan casing has an annular metallic inner shell with a plurality of holes formed therethrough. An acoustic absorber, such as a cellular resonator, is disposed around the inner shell.